The use of hydroxybenzoic acids for the preparation of polyesters and copolyesters is known. For example, U.S. Pat. No. 2,600,376 shows the preparation of polyesters of m- and p-hydroxybenzoic acid. Copolyesters are similarly obtained employing hydroxyaromatic dibasic acids, such as 2-hydroxy- terephthalic acid, in U.S. Pat. No. 3,887,468. .beta.-Oxyethoxy vanillic acid is used for the production of copolyesters in U.S. Pats. No. 3,056,761 and 3,288,755.
It is also well known to use halogenated (either chlorine or bromine) reactants for the preparation of polyesters and copolyesters having improved flame retardant properties. Numerous halogenated compounds containing hydroxyl and/or carboxyl functionality have been suggested for this purpose. U.S. Pat. No. 3,752,867, for example, discloses fireproof unsaturated polyester resins containing halogenated salicyclic acid. Fire-resistant unsaturated polyesters derived from bromine-containing compounds such as 3,5-dibromo-4-hydroxybenzoic acid are also disclosed in Japanese Pat. No. 74 11,875.
Even though it is recognized that brominated hydroxy and carboxy compounds are effective flame retardants for polyesters there are serious drawbacks associated with the use of such brominated derivatives in the preparation of high molecular weight polyesters useful as fibers and filaments because of the high polymerization temperatures (generally in excess of 250.degree. C and often as high as 320.degree. C) required. At these high temperatures most brominated materials tend to decompose and thus impart undesirable discoloration and, in some instances where the decomposition is pronounced, significantly detract from the overall physical properties of the polymer. Brominated compounds have therefore typically been limited to use in the preparation of unsaturated polyesters resins where much lower reaction temperatures (typically less than 200.degree. C) are encountered.